1. Field of the Invention
This invention pertains to the field of electronic circuit boards used for interconnecting electronic components into functional subassemblies, and more specifically is directed to circuit boards having conventional single or multi-layer conductive traces in combination with an optical fiber interconnect layer embedded between layers of the circuit board.
2. State of the Prior Art
The rapid increase in data transmission and data processing rates brought about by broadband communications and interactive telecommunication and computer services creates a need for increased interconnection density and capacity in electronic equipment. This need has led to a growing reliance upon optical fiber as a replacement for traditional wire transmission lines, and has resulted in the almost complete replacement of copper wire with optical fiber for long distance transmission because of lower transmission losses and superior bandwidth characteristics. Optical fiber transmission can also improve system performance if applied at short distances, as between physically adjacent equipment racks and cabinets, or between offices in a given building. However, the benefits of optical fiber transmission extend to even shorter distances, as at intra-board level among integrated circuits and other components on a single circuit board, and at the intra-module level for interconnecting for example very large scale (VLSI) and ultra large scale (ULSI) integrated circuits and chip subassemblies in a single electronic module operating at Gigabyte speeds.
Advantages of optical interconnects over electrical conductors at the board and module level include immunity to electromagnetic interference (EMI) or electrical noise, electrical isolation of interconnected components, far less frequency dependent signal degradation, and higher possible density of interconnects due to lack of cross-talk between closely spaced, fine conductors.
Current efforts at providing optical Interconnects at the circuit board level are exemplified by optical flex technology such as the Optical Flex circuitry marketed by Advanced Interconnection Technology, LLC of Islip, N.Y. and the optical flex foil developed under the Apollo Demonstrator project at the Micro Interconnect Research Center of L M Ericsson, Stockholm, Sweden and described in Ericsson review, No. 2, 1995, vol. 72. In general these optical interconnects involve arranging lengths of optical fibers in a desired pattern customized to the intended application, laminating the optical fibers between sheets of a flexible foil and applying appropriate connectors and terminations to the fiber ends. The lamination holds both the fibers and the connectors in the desired layout. The flex foil interconnect is assembled to a conventional rigid circuit board simply by plugging the connectors to corresponding mating connectors on the circuit board. Mechanical supports may be provided on the circuit board for stabilizing the flex foil in place rather than relying on the fiber connectors alone for this purpose. The flex foil is typically supported in spaced relationship above the electrical components on the board. The resulting assembly tends to be awkward, costly and less than fully reliable due to reliance upon optomechanical connectors and the need to mechanically assemble the optical flex foil to the circuit board.
It has been also suggested in the literature that the flex foil be laminated or bonded to rigid circuit board thereby to integrate optical and electrical interconnects. Even if so laminated, however, current fiber flex foil approaches to the application of optical interconnects at the circuit board level still call for the use of optical connectors and terminations of the fibers and in this regard fall short of true integration of optical and electrical board level interconnections. Furthermore, the laminated flex foil will typically interfere with free layout of electrical parts on the circuit board.
A continuing need exists for better integrated, lower cost and more reliable optical interconnects for electronic circuit boards.